1. Field of the Invention
This invention relates to a method of and apparatus for inspecting coatings on a substrate, particularly reflective coatings on a flat glass substrate.
2. Discussion of the Presently Available Techniques
Developments in the glass coating art and in particular, in the area of architectural glass, have led to a variety of techniques for depositing thin reflective coatings on a glass substrate. Both discrete glass sheets and a moving glass ribbon have been coated to provide the desired reflectivity, as well as aesthetically pleasing color. For example, U.S. Pat. No. 3,660,061 teaches a method of applying a pyrolytic coating to a moving glass ribbon, and Canadian Pat. No. 756,053 teaches a method of coating discrete glass sheets.
As can be appreciated, any coating must be uniformly applied to a moving ribbon so that discrete sheets cut from the ribbon will have a consistently uniform appearance when installed side by side, e.g. in a curtainwall construction. Individual sheets as well must be free from visible color variations and defects. For this reason, uniform thickness of the coating is important to achieve the desired uniform appearance. Variations in coating thickness which appear as color and shading variations in the installed sheet are not readily visible to the naked eye observing either the moving coated ribbon, or the sheets cut from the ribbon, under normal ambient lighting conditions in the production facility. For this reason, some inspection technique is necessary to assure the quality of the coated glass with respect to uniformity of coating thickness.
One technique available for the inspection of such coatings on the production line includes a device such as that sold under the trade name of Chromoscope by the Millitron Company of Pittsburgh, Pa. This device is mounted in a stationary position above a moving ribbon of coated glass and incident light is directed at a selected angle to a point on the coated surface. Light reflected from the point on the surface is received by the device which measures the reflectance of blue light and the reflectance of red light and determines the thickness of the film as a function of the ratio of these measured reflectances. The flow rate of the filming composition is then adjusted, if necessary, to compensate for any variation from desired thickness.
U.S. Pat. No. 3,395,278 teaches a method of measuring the thickness of a transparent metallic oxide coating on articles by using an ultraviolet light source and measuring the degree of reflectivity of light from a single point on the coating, and converting the degree of measured reflectivity into a reading of coating thickness.
Although it might be possible to adapt either of these methods for scanning the entire surface of a moving ribbon, because of their inherent limitations, this is impractical. This is so because both methods as practiced operate on a beam reflected from the single point on the coated surface. As can be appreciated, providing facilities for scanning the entire surface in a pointwise fashion is slow, and the apparatus for accomplishing the task would be complex and costly.
Another method used to inspect coatings on glass panels is to visually inspect a sample of the coated glass outdoors under natural sunlight conditions. Generally, this method is used in addition to an online reflectance ratio monitoring device similar to the type discussed above, and it is used to detect variations which may have escaped detection by the online device, but would appear as color variations in a coated glass panel after installation. This technique is primarily used to detect the defect known as "banding ", which is the term used for variations in coating thickness which consist of alternating fairly wide bands of thick and thin coating. Although this method allows visual inspection of an entire sample surface and can reveal banding, it has limitations. For example, this technique cannot be used for online inspection and furthermore, samples are not easily inspected on cloudy or dark days.
As can now be appreciated, it is desirable to have a simple, fast method of inspecting coating thickness on a substrate, e.g. a glass panel, that does not have the limitations of the presently available techniques.